1. Field of the Invention
This invention relates to electronic photocopiers, electrophotographic laser beam printers, and other image forming apparatus which form electrostatic latent images and develop them with toner.
2. Related Background Art
A developing sleeve carrying a thin layer of toner is installed as a developing unit of the image forming system, facing an image carrier with a greater distance therebetween than the thickness of the thin layer. Then, bias voltage having an AC component is applied to the developing sleeve. The vibrating electric field formed between the image bearing member or image carrier and sleeve causes toner to reciprocate between them. Thus, latent images formed on the image carrier are made visible by developing apparatus. This kind of developing systems have been known from, for example, U.S. Pat. No.4,395,476.
In those developing systems, in order to detect the remaining amount of toner, an electric conductor is installed in the developing unit facing the sleeve to use induced voltage produced in the conductor via toner according to the voltage applied to the developing sleeve. In this popular method, the change in capacitance between the developing sleeve and conductor which results from the variation in the amount of toner interposing between them is detected with voltage. Then, the voltage is compared with a reference voltage.
In FIG. 1, bias voltage, having an AC component the power supply 3 has developed (a DC voltage component is superimposed), is applied to the developing sleeve which rotates in arrow direction to carry toner 4 and applies the toner to the electronic photosensitive material 0 rotating in arrow direction. The detecting conductor 2 and developing sleeve 1 correspond to pole plates of capacitor, respectively. The capacitance varies with the amount of toner 4 interposing between the detecting conductor and developing sleeve. A signal voltage developed in the detecting conductor 2 is converted into a DC voltage V.sub.DC, through the rectification circuit 5 and capacitance-voltage conversion circuit 6. The voltage V.sub.DC value corresponds with the said amount or the amount of toner in the developing unit. The reference voltage generation circuit 7 inputs reference voltage V.sub.s and detected DC voltage V.sub.DC into the comparison ciucuit 8 to check which value is larger. When the remaining amount of toner becomes lower than the given amount and voltage V.sub.DC becomes less than V.sub.s, the remaining amount detecting indication circuit 9 is activated to indicate warning information.
The DC voltage V.sub.DC is affected with the variation in peak-peak voltage of AC voltage V.sub.pp applied to the developing sleeve 1. Normally, the V.sub.PP value the AC power supply 3 develops varies by approximately +/-10% for its performance sake. Therefore, even if the amount of toner is constant, the DC voltage V.sub.DC detected varies with the AC bias voltage variation. Therefore, warning information prompting for comparison with the reference voltage V.sub.s is output with the different amounts of toner. This is a significant drawback.
A well-known alternative method is that a capacitor having a reference capacitance is used to form a reference voltage signal, so that the detecting efficiency of the remaining amount will not be affected even if AC bias voltage varies (for instance, Japanese Patent Laid-open No. 63-210870). FIG. 2 shows the outline of the system.
In FIG. 2, the reference voltage generating circuit is equipped with a capacitor 10a having capacitance C.sub.s similar to the amount of toner to be detected. When the power supply 3 applies bias voltage containing an AC component to the developing sleeve 1, said bias voltage is applied to one electrode of the capacitor 10a at the same time. Then, voltage occurs at another electrode of the capacitor 10a, which is converted into DC reference voltage V.sub.s through the rectification circuit 10b and capacitance-voltage conversion circuit 10c. The reference voltage V.sub.s and detecting voltage V.sub.DC are compared in the comparison circuit 8 to detect amount of toner. This method has an advantage that voltage variation in peak-peak voltage of bias voltage V.sub.pp, if it occurs, would affect voltages V.sub.DC as well as V.sub.s. Therefore, variation in peak-peak voltage V.sub.pp is ignorable.
The developing unit 11 which incorporates a detecting conductor 2 (which is also equipped with the said sleeve 1) is installed away from the toner remaining amount detecting circuit 12 comprising the said circuits 5, 6, 8, 10a, 10b, and 10c. On the other hand, the voltage signal corresponding to the amount of toner in the developing unit 11 which is induced into the detecting conductor 2 is a weak signal. Therefore, a shielding wire 16 (FIG. 3) is used to prevent noise from mixing with the weak signal, so that the signal can be transmitted to the toner remaining amount detecting circuit 12. The shielding wire 16 consists of a lead wire 13 for the said signal, an insulation layer 15 made from synthetic resin which encapsulates the lead wire 13, and an electric shielding member 14 which encapsulates the insulation layer 15. The shielding member 14 is usually made from texture of fine metallic lines. Whatever is made into the shielding member 14, it is electrically grounded, However, there is a drawback that floating capacitance developed between lead wire 13 and the shielding member 14 affects detection voltage.
In FIG. 3, the developing unit 11, or the assembly of the developing unit and photosensitive drum D is detachable from the imaging system main unit 17. The detachable connector 18 connects the detecting conductor 2 and lead wire 13. The detachable connector 19 connects the sleeve and a lead wire 20 coming from the power supply 3 which applies bias voltage.